FIG. 10 is a graph illustrating temperature dependence of current and voltage characteristics of a PN junction element. A logarithmic expression in which a horizontal axis thereof indicates a forward voltage Vbe [V] of the PN junction element and a vertical axis thereof indicates a forward current Ie [A] of the PN junction element is illustrated. The PN junction element is, for example, a bipolar transistor. The voltage Vbe is a voltage between a base and an emitter of the bipolar transistor, and the current Ie is an emitter current. Characteristics T1 to T6 indicate current and voltage characteristics in accordance with temperature. The characteristic T1 is when it is −40° C., the characteristic T2 is when it is 0° C., the characteristic T3 is when it is 25° C., the characteristic T4 is when it is 55° C., the characteristic T5 is when it is 85° C., and the characteristic T6 is when it is 125° C. In the case when the same current Ie flows, as temperature rises, the voltage Vbe lowers. A voltage V1 illustrated by a square mark indicates a voltage for allowing the current Ie, which is approximately 4×10−9 [A], to flow, and as temperature rises, it lowers. A voltage V2 illustrated by a circle mark indicates a voltage for allowing the current Ie, which is approximately 5×10−6 [A], to flow, and as temperature lowers, it rises. Here, the voltage V1 has high temperature dependence with respect to the voltage V2.
FIG. 11 is a graph illustrating a relation between a voltage of the PN junction element and temperature. A horizontal axis thereof indicates temperature and a vertical axis thereof indicates a voltage. The voltage V2, as illustrated in FIG. 10, lowers as temperature rises. On the other hand, a voltage V2−V1 rises as temperature rises.
A reference voltage generating circuit may generate a reference voltage that does not depend on temperature by using two PN junction elements having different current densities. A forward voltage of the first PN junction element is V1, and a forward voltage of the second PN junction element is V2. When the voltages V1 and V2 are in a relation of V1<V2, the reference voltage generating circuit generates a reference voltage Vref expressed by the reference voltage Vref=V2+α×(V2−V1). As illustrated in FIG. 11, if a coefficient α is selected appropriately, approximately 1.25 V as the reference voltage Vref that does not depend on temperature may be obtained.
In recent years, lowering voltage is required, and therefore, a reference voltage generating circuit operating at a voltage lower than 1.25 V is required. As one of low voltage techniques, Patent Document 1 that is described below exists. In Patent Document 1, an output voltage to be a reference voltage is set such that a voltage that is a times as large as a voltage V2−V1 being a voltage difference between a base and an emitter of bipolar transistors having different current densities and a voltage that is one-βth (β>1) of a voltage V2 between the base and the emitter of the bipolar transistor are added. That is, the reference voltage generating circuit generates the reference voltage Vref expressed by Vref=V2/β+α×(V2−V1).
However, the above reference voltage generating circuit has a problem in which a circuit scale is increased. For example, in an example of operation performed by a voltage illustrated in Patent Document 1, six operational amplifiers are used, resulting that there exist problems that an area occupied in a semiconductor chip and power consumption are increased.
Further, in Patent Document 2 that is described below, there is discussed a reference voltage generation circuit including: a first current generation circuit generating a current proportional to a difference between a first forward voltage of a PN junction and a second forward voltage of a PN junction having a different current density; a second current generation circuit generating a current to equalize a voltage proportional to the current obtained from the first current generation circuit and the first forward voltage; and a voltage addition circuit adding a voltage proportional to the current obtained from the second current generation circuit and the first forward voltage.
Patent Document 1: Japanese Laid-open Patent Publication No. Hei 05-251954
Patent Document 2: Japanese Laid-open Patent Publication No. 2004-192608